1. Field of the Invention
The present invention relates to an integrated resistor, a phase-change memory element including this resistor, and a process for the fabrication thereof.
2. Description of the Related Art
As is known, phase-change memory elements, or PCM elements, exploit the characteristics of a class of materials able to change between two phases having distinct electrical characteristics. For example, these materials may change from an amorphous, disorderly phase to a crystalline or polycrystalline, orderly phase, and the two phases are associated to considerably different values of resistivity.
At present, alloys of elements of group VI of the periodic table, such as Te or Se, referred to as chalcogenides or chalcogenic materials, can advantageously be used in phase-change cells. The chalcogenide that currently offers the most promise is formed by a Ge, Sb and Te alloy (Ge2Sb2Te5) and is widely used for storing information in overwritable disks.
In chalcogenides, the resistivity varies by two or more orders of magnitude when the material passes from the amorphous phase (more resistive) to the crystalline phase (more conductive) and vice versa. The characteristics of the chalcogenides in the two phases are shown in FIG. 1. As may be noted, at a given read voltage, here designated by Vr, there is a resistance variation of more than 10.
Phase change may be obtained by locally increasing the temperature, as shown in FIG. 2. Below 150° C. both phases are stable. Above 200° C. (nucleation start temperature, designated by Tx), fast nucleation of the crystallites takes place, and, if the material is kept at the crystallization temperature for a sufficient length of time (time t2), it changes its phase and becomes polycrystalline. To bring the chalcogenide back into the amorphous state, it is necessary to raise the temperature above the melting temperature Tm (approximately 600° C.) and then to cool the chalcogenide off rapidly (time t1).
From the electrical standpoint, it is possible to reach both the critical temperatures, namely crystallization and melting temperature, by causing a current to flow through a resistive element which heats the chalcogenic material by Joule effect.
The basic structure of a phase-change memory element 1 which operates according to the principles described above is shown in FIG. 3 and comprises a resistive element 2 (heater) and a programmable element 3. The programmable element 3 is made of a chalcogenide and is normally in the polycrystalline state in order to enable a good flow of current. One part of the programmable element 3 is in direct contact with the resistive element 2 and forms the area involved in the phase change, hereinafter referred to as phase-change portion 4.
If an electric current having an appropriate value is made to pass through the resistive element 2, it is possible to heat the phase-change portion 4 selectively up to the crystallization temperature or to the melting temperature and to cause phase change. In particular, if a current I is made to pass through a resistive element 2 having resistance R, the heat generated is equal to I2R.
At present, the resistive element 2 is obtained by deposition—using PVD (Physical Vapor Deposition), Reactive PVD and CVD (Chemical Vapor Deposition)—of materials having a resistivity of between a few hundred μΩcm and a few ten mΩcm. The material thus obtained has a substantially homogeneous resistance in all directions.
The memory element described above is disadvantageous since it has a high dissipation on account of the high resistance of the resistive element, even if the portion useful for generating the phase change heat for the memory element 1 is only one part of its volume. A high level of dissipation may, in fact, be harmful for the materials and components integrated in the chip. The problems associated with dissipation of the entire resistive element moreover impose design constraints on the values of resistivity that can be used for the resistive element, as well as on the programming currents and voltages, giving rise to high levels of consumption.